Add setting to allow manually enabling CursorHider

After the discussion in #1348, cursor hiding was completely disabled. Since it still has some utility, I [proposed there](https://github.com/philc/vimium/issues/1359#issuecomment-67738687) that we should have some way of enabling it without having to modify the source code. This PR gives such a method.

With this PR, `CursorHider` can be enabled by running `Settings.set("enableCursorHider", true)` in the background page.